Dark exciton signatures in time-resolved photoluminescence of single quantum dots
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Dark exciton signatures in time-resolved photoluminescence of single quantum dots Jason M. Smith1, Paul A. Dalgarno1, Richard J. Warburton1, Brian D. Gerardot2 and Pierre M. Petroff2 1 School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K. 2 Materials Department and QUEST, University of California, Santa Barbara, California 93106, U.S.A. ABSTRACT Time-resolved photoluminescence of single charge tuneable quantum dots allows us to probe the differences in recombination dynamics between neutral and negatively charged excitons. We find that the luminescence decay from a neutral exciton contains a second lifetime component of several nanoseconds that is not present in the luminescence from singly or doubly charged excitons. We attribute the slowly decaying component to excitation cycles in which the initial exciton formed in the dot is dark, with angular momentum M = 2, and which subsequently scatters into the bright state with M = 1. The nature of the scattering mechanism is revealed by the dependence of the lifetime on the electrical bias applied across the charge-tuneable device. That the lifetime changes by an order of magnitude within a short bias range implies that the dark-to-bright transmutation does not occur through a simple spin flip. Rather it appears to come about by the dot briefly entering a higher energy charging state which allows exchange of the existing electron with another from the ntype contact region. We model the lifetimes and relative intensities of the two decay components using a simple rate equation analysis.
INTRODUCTION Dark excitons are electron-hole complexes which do not couple to the optical field and are thus impossible to probe directly using absorption or luminescence techniques. As they do not recombine radiatively, dark excitons can be much more long-lived than bright excitons, offering potential for use in memory storage or quantum information processing applications. Studies of dark excitons generally rely upon some form of symmetry breaking to allow access with an optical interaction.1,2 The lowest energy excited state of most semiconductor quantum dots is dark, with the spin of the excited electron aligned with the hole resulting in a total angular momentum of M=2. In InGaAs Stranski Krastanow quantum dots, which are the subject of this study, this state lies approximately 0.5 meV below the lowest bright state, with M=1, where the splitting is a result of the electron-hole exchange interaction. In an isolated dot, little communication is expected between the dark and bright states, as such a process requires a spin flip of one of the carriers that is not facilitated by acoustic phonon scattering.3 In this paper we report time-resolved photoluminescence (TRPL) measurements of single Stranski Krastanow quantum dots embedded in charge tuneable heterostructures.4 We compare TRPL from the neutral exciton (e:h, labelled X0) with that from the singly charged exciton (2e:h,
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labelled X1-) and find evidence for an electrically cont
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